The creation of human-animal hybrid embryos -proposed as a way to generate embryonic stem cells without relying on scarce human eggs - has met with legislative hurdles and public outcry. But a paper published this week suggests that the approach has another, more fundamental problem: it may simply not work. Robert Lanza of Advanced Cell Technology, a stem-cell company based in Los Angeles, California, and his colleagues show that in their labs, early-stage human-cow, human-mouse and human-rabbit hybrid embryos fail to grow beyond 16 cells (Y. Chung et al. Cloning Stem Cells doi:10.1089/ clo.2009.0004; 2009). The hybrid embryos also failed to properly express genes thought to be critical for pluripotency - the ability to develop into a wide variety of cell types. Lanza and his co-workers created their hybrid embryos using a process called somatic-cell nuclear transfer, a technique made famous when it was used to create Dolly the cloned sheep in 1996. This time, the researchers replaced the nuclei of human, cow, mouse and rabbit eggs with nuclei from human non-sex, or somatic, cells.
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